The present invention relates generally to hydrostatic transmission control systems, and more particularly, to such a control system which can function as an antistall control, to vary the input to output speed ratio of a hydrostatic transmission and prevent overloading of the primer mover.
Hydrostatic transmission control systems, including anti-stall controls, are now well known in the art. Such controls have conventionally been either hydraulic or a combination of hydraulic and mechanical. The known, basic anti-stall control was merely an arrangement which reduced the pressure of control fluid communicated from the charge pump to the main system control means. The reduction in charge pressure in such a control is proportional to the drop in engine speed, below the reference speed.
Although such basic anti-stall controls were fairly simple, various operational shortcomings of such systems resulted in further development and system sophistication. For example, in U.S. Pat. No. 3,886,741, assigned to the assignee of the present invention, the anti-stall control includes a rate control valve which limits the rate at which the swashplate would come back to commanded stroke, after the impending stall condition was alleviated, to prevent the condition known as "cycling".
As a result of this and other similar developments in hydraulic anti-stall controls, it was possible for those skilled in the art to produce an anti-stall control which could be made to operate satisfactorily on a given vehicle, and under a given set of conditions. However, because of the complexity of such systems, extensive setup and adjustment of the system was required after installation on the vehicle, and even then, repeatability in performance of the system could not always be achieved.
Accordingly, it is an object of the present invention to provide an anti-stall control which is relatively simple and inexpensive compared to known anti-stall controls, but which has a substantially improved repeatability of performance, with minimum setup and adjustment required.
Another drawback of the known anti-stall controls was that, partially because of their complexity, such controls are not readily adaptable to perform other control functions, except by means of additional linkage and components which would have merely compounded the above-described problems of difficulty of setup and lack of repeatability.
Accordingly, it is another object of the present invention to provide an anti-stall control which is capable of performing other control functions, either simultaneously with its anti-stall function, or as an alternative thereto.
It is a related object of the present invention to provide a control system which achieves such versatility merely by means of minor modification to the electronic logic portion of the control system.
The above and other objects are accomplished by the provision of an improved control system for a hydrostatic transmission of the type including a prime mover driven variable displacement pump; fluid operable means for varying the displacement of the pump, the fluid operable means defining first and second fluid pressure chambers; a source of pressurized control fluid; main control means operable to control the flow of pressurized control fluid from the source to the fluid operable means; the source of control fluid, the main control means and the first fluid pressure chamber defining a first control fluid path; and the second fluid pressure chamber, the main control means and a fluid reservoir defining a second control fluid path. The improved control system includes an auxiliary control valve means operable to communicate control fluid from the first control fluid path to the second control fluid path. The auxiliary control valve means includes an on-off, electro-hydraulic valve disposed in the control conduit means to prevent fluid communication therethrough in response to an electrical input signal in one condition, and to permit fluid communication therethrough in response to an electrical input signal in another condition. A control logic means is operable in response to variations in an input condition to alternate the electrical input signal between the one condition and the other condition at a duty cycle determined by the deviation of the input condition from a normal operating condition.